The present invention relates to a core drill, in particular for oil prospecting, comprising a core bit, an outer tube supporting the core bit so as to drive its rotation for core drilling, and an inner tube mounted in the outer tube so that it can receive a core sample cut by the bit. A flow space provided between the outer and inner tubes is intended for the passage of a coring fluid to be conveyed to the bottom of a hole during core drilling.
There is a constant need to improve the operation of core drills, for example by adding to them auxiliary devices which improve the reliability of the operations of taking hold of a core sample which has been cut around its periphery and which has to be detached from the bottom of the bore hole in order to bring it up to the surface. The way in which the auxiliary device or devices themselves work must, for its own part, be controllable from the surface so that they can act at the desired moment and in the intended way, with the lowest possible risk of losing or damaging the core sample, damaging the core drill, etc, given the enormous cost involved in terms of labor and time for performing such core drilling operations which have therefore to be successfully completed as quickly as possible.
The object of the present invention is to provide simple and effective means for controlling these auxiliary devices without introducing sophisticated and/or delicate mechanisms in the core drill but making best use of the possibilities, known as being such, offered by the use of modifications to the pressure of the core drilling fluid.
To this end, in the core drill of the invention, means for restricting the passage of core drilling fluid are arranged in the flow space at the front end side of the inner tube, viewed according to a direction of advance of a core sample, and in addition, control means are provided for adjusting, from the surface, the restricting means so as to substantially increase the pressure of said fluid upstream of the restricting means.
This increase in the pressure of the core drilling fluid can thus be exploited directly and effectively at the front end of the core drill in order to actuate the aforementioned auxiliary devices there.
According to one embodiment of the invention, the passage-restricting means comprise for this purpose an element of inner tube and an element of outer tube which elements collaborate to produce the adjustable restriction, and the control means are designed to move the inner and outer tubes one with respect to the other.
In another embodiment of the invention, the passage-restricting means comprise for this purpose an element of inner tube and/or an element of outer tube and an auxiliary element designed to collaborate with the element of inner tube and/or the element of outer tube with a view to adjusting the restriction. The control means may therefore be designed for this purpose to move the inner and/or outer tubes and/or the auxiliary element one with respect to the other.
As a preference, in the core drill of the invention, at least one of said elements of inner and/or outer tubes is an annular boss of which one peripheral surface facing toward the other tube element collaborates with a peripheral surface of this other element to form the restriction.
Advantageously, the control means may be designed to move the inner and outer tubes longitudinally one with respect to the other with a view to adjusting the restriction, the inner tube preferably being moved towards the front end of the core drill during this adjustment. For this purpose, the control means may comprise, on the rear end side of the inner tube, a cylinder and piston assembly with a relatively limited travel, one of the piston and cylinder being secured to the inner tube and the other being secured to the outer tube. A catch may be fitted so as to be able to lock the piston in the cylinder in a position corresponding to a chosen minimum restriction (relatively large passage). In addition, the catch may be fitted so as to unlock the piston from the cylinder when a determined pressure of core drilling fluid, higher than the core-drilling pressure, is applied to the catch. The limited relative travel of piston relative to cylinder as a result of a pressure which is still higher, brings then the restricting means from the front end of the inner tube into a position of chosen maximum restriction (relatively small passage).
In the core drill of the invention, one type of auxiliary device may comprise, arranged coaxially in the front end of the inner tube, a sleeve with deformable wall which, in the undeformed condition, allows a core sample to pass, and a substantially impervious annular chamber contained between the inner tube and the wall of the sleeve, this chamber being in communication with the core drilling fluid passing through the flow space between the outer and inner tubes. The wall of the sleeve is chosen to deform toward the inside of the inner tube until the internal space thereof is substantially closed up and/or closed so as to retain a core sample located therein, under the pressure of said fluid which pressure is obtained in the annular chamber after an aforementioned adjustment of the pressure by the chosen maximum restriction.
Another type of auxiliary device for the core drill of the invention may comprise, arranged coaxially in the front end of the inner tube, a sliding ring allowing the passage of a core sample and mounted in the manner of a piston in a substantially impervious annular chamber contained between the inner tube and this ring, this chamber being in communication with the core drilling fluid passing through the flow space between the outer and inner tubes. The sliding ring is then designed to, on the one hand, occupy a first position slid towards the front end of the core drill and be preferably locked therein by a catch, and, on the other hand, be slid into a second position away from the front end of the core drill under the pressure of said fluid which pressure is obtained in the annular chamber after the increase in the aforementioned pressure by the chosen maximum restriction, after any catch there might be has released the ring under the action of said pressure on the ring.
Further details and particulars of the invention will become apparent from the secondary claims and from the description of the drawings which are appended to this description and which illustrate, diagrammatically and in longitudinal section, with cutaway and possibly different scales, and as non limiting examples, some advantageous embodiments of core drills according to the invention.